1. Field of the Invention
This invention relates to a buried type of semiconductor laser and in particular to a semiconductor laser having excellent elevated-temperature and high-power properties.
2. Description of the Related Art
Recently, a high-power semiconductor laser has been needed for an optical CATV, a light source for EDFA excitation or an OTDR.
For meeting such needs, for example, ELECTRONICS LETTERS, VOL.18, NO.22, 1982 has suggested a DC-PBH (double channel Planar buried hetero-structure) laser, where a leakage current is recombined in recombination layers 2 having the same composition as an active layer 1, formed on both sides of the active layer as shown in FIG. 11, to minimize turn-on of a block layer due to a p-n junction for attempting to improve its elevated-temperature and high-power properties.
However, there are still needs for higher level of elevated-temperature and high-power properties, and thus extensive studies have been conducted for meeting the needs.
For example, JP-A 6-283800 has disclosed a technique that a concentration of a block layer and a channel width are designed to optimize distribution between a leakage current flowing a p-n junction and a current flowing a recombination layer for improving elevated-temperature and high-power properties of a DC-PBH (double channel Planar buried hetero-structure) laser. In the laser, a concentration of a p-type dopant in a p-type current block layer 4 is elevated and a channel width is narrowed to increase a current flowing a recombination layer 2 as shown in FIG. 2, while relatively reducing a current flowing a channel 3. Thus, a semiconductor laser has been suggested, in which a total leakage current at a high-power operation may be reduced to improve its high-power properties.
The technique described in the above publication, however, has the following problems.
First, it may not provide adequate high-power properties. In general, Zn is used as a dopant for a p-type current block layer. As the concentration of the dopant Zn increases, more Zn atoms are diffused into an active layer from a p-type current block layer during forming the p-type current block layer, resulting in increase of internal loss in the active layer. It may cause reduction in an efficiency and deterioration in high-power properties of the laser.
Secondly, an effect of increase of the dopant concentration in the p-type current block layer may be substantially saturated about at 1.times.10.sup.18 cm.sup.-3 because increase of the dopant concentration in the block layer may increase a leakage current flowing the recombination layer while saturating a carrier-recombination rate in the recombination layer. In other words, there is a certain limit for improvement by increasing the dopant concentration.
Thirdly, a laser may become less reliable. As described above, an elevated dopant concentration in the p-type current block layer may increase contamination of the active layer with the dopant, leading to making the laser less reliable.